Only fill methods with 0xFE on debug builds

The GC scans classes without holding any locks, we can not fill
the methods if it is running. Added a GC critical section to address
this. Fixes random crash when scanning classes' methods.

Only for debug builds to not hurt performance.

(cherry picked from commit 22bd2a1b5ec2a5038cc3ae1964781f30aef0315f)

Bug: 28699001
Change-Id: If96155eaf3fc0e6df31f57dcf32fbd4063b09345
5 files changed